1. Field
The present invention relates to a travelling wave antenna having low profile height or thickness while providing wideband operation. The antenna comprises a plate waveguide in which a transverse electromagnetic transmission (TEM mode) is propagated.
The invention further relates to methods of producing such waveguide with the low profile height and wide bandwidth at relatively minimal cost.
2. Description of Related Art
The use of waveguides for a travelling wave antenna is well known. Such antennas are well suited to consumer applications where the overall thickness of the waveguide must be kept to an absolute minimum. For example, for automotive applications, it is desirable to install the antenna within the roof of the vehicle. However, the antenna must not be visible and this imposes a rigid constraint on the overall thickness of the travelling wave antenna to about one inch.
FIGS. 1 and 2 diagrammatically illustrate the construction of a waveguide 1 of a travelling wave antenna which comprises upper and lower conductive plates 2 and 3 respectively, and a dielectric material 4 sandwiched between the plates. A line source (not shown) is coupled to an inlet end of the waveguide 1 to produce the propagated wave therein. The upper plate 2 is provided with a number of apertures 5 extending transversely thereacross almost the full width of the upper plate 2. The apertures 5 serve as a means for radiating energy and their design is especially crucial to achieve the desired performance of the antenna while maintaining the low profile or thickness t of the waveguide 1. In FIGS. 1 and 2, the apertures 5 have been shown as rectangular slots of constant width w. The thickness x of the upper plate 2 is about λ/4, where λ is the wavelength of the incident energy.
In a first known embodiment shown in FIG. 3, in order to adjust the radiation energy of the waveguide, rectangular apertures 5A, 5B of different widths and heights are provided along the length of the waveguide. The different heights of the apertures are obtained by forming a step in the top plate 2a at each aperture. By adjusting the width and the height of the apertures 5A and 5B, various pattern amplitudes and phase shapings can be obtained. However, relatively narrow band slot impedance characteristics are produced. In addition, there is a limit to the impedance values that can be obtained and this may not be sufficient to provide the desired radiation performance. Consequently, this antenna construction often results in low bandwidth.
FIG. 4 shows an improved embodiment in which apertures of constant height are provided and the apertures have varied widths. Specifically, the top plate of the waveguide is formed by lower and upper plate members 2′ and 2″ respectively, each formed with respective rectangular apertures 5′ and 5″.
By adjusting the width of the rectangular apertures 5′, 5″ the radiation energy of the waveguide can be adjusted. The plate members 2′ and 2″ each have a thickness of approximately λ/4. The rectangular apertures 5′, 5″ formed in the plate members 2, 2″ are rectangular slots having parallel faces. The width of the apertures 5′, 5″ can be varied along the length of the waveguide. The apertures 5′ and 5″ are aligned with one another and provide an overall stepped aperture having an inner aperture width formed by apertures 5′ and a larger outer aperture width formed by apertures 5″. Although this embodiment provides apertures with constant height and a wider range of aperture impedance, the overall height of the top plate is doubled which makes the waveguide unusable where thickness is critical.
Various additional aperture designs in waveguides are known and, by way of examples, U.S. Pat. Nos. 5,266,961 and 5,349,363 illustrate antennas in which the radiating apertures are formed by transverse stub elements formed on the top plate.